Bus bar arrangement for an electrically heated vision unit

Abstract

An electrically heated vision unit, such as a windshield, has bus bars extending substantially parallel to, and spaced from, each other in contact with an electrically conductive film provided on a surface of a transparent substrate. One bus bar extends laterally beyond the corresponding end of the opposite bus bar, an end portion at such corresponding end of the opposite bus bar being recessed away to avoid excessive current density at such recessed end portion.

We claim: 
     
       1. A substantially transparent, electrically heated vision unit comprising: a self-supporting substrate;   an electrically conductive film partially covering a surface of said substrate; and   a pair of opposed, elongate, electrically conductive bus bars, each having an elongate inward edge which faces the other and is in electrically conductive, overlapping contact with said film, a first bus bar of said pair of electrically conductive bus bars being spaced from, and substantially parallel to, a second bus bar of said pair of electrically conductive bus bars, one bus bar of said pair of electrically conductive bus bars extending from one end which forms an electrical junction with an electrically conductive lead, which lead is electrically isolated from said film other than through said first bus bar, to a distal end, said electrical junction being on an area of said surface of said substrate not covered by said film, said one bus bar intersecting said film at an intersection mediate said one end and said distal end, said inward edge of said one bus bar having a minor recess at said intersection away from the opposed bus bar; wherein an end of said second bus bar extends beyond a corresponding end of said first bus bar, and elongate end portion of the inward edge of said first bus bar at said corresponding end being step recessed away from said second bus bar.     
     
     
       2. The electrically heated windshield unit of claim 1 wherein said electrically conductive film only partially covers said surface of said substrate. 
     
     
       3. The electrically heated vision unit of claim 1 wherein said substrate is glass and said electrically conductive film comprises a film stack wherein an electrically conductive metal film is sandwiched between dielectric films. 
     
     
       4. The electrically heated vision unit of claim 3 wherein said surface of said substrate with said film thereon is a surface of a first self-supporting ply which is laminated to a second self-supporting ply by a flexible polymeric laminating layer between said first and second plys. 
     
     
       5. The electrically heated vision unit of claim 1 wherein the recessed end portion is step recessed, having a lateral dimension equal to about 1% to about 6% that of said first bus bar. 
     
     
       6. The electrically heated vision unit of claim 1 wherein the recessed end portion is step recessed, being recessed a dimension R which is equal to about one-half to about one-eighth the magnitude of the lateral dimension L of said recessed end portion. 
     
     
       7. A substantially transparent, electrically heated vision unit comprising: a self-supporting substrate;   an electrically conductive film partially covering a surface of said substrate; and   a pair of opposed, elongate, electrically conductive bus bars, each having an elongate inward edge which faces the other and is in electrically conductive, overlapping contact with said film, a first bus bar of said pair of electrically conductive bus bars being spaced from, and substantially parallel to, a second bus bar of said pair of electrically conductive bus bars, said first bus bar of said pair of electrically conductive bus bars extending from one end which forms an electrical junction with an electrically conductive lead, which lead is electrically isolated from said film other than through said first bus bar, to a distal end, said electrical junction being on an area of said surface of said substrate not covered by said film, said first bus bar intersecting said film at an intersection mediate said one end and said distal end, said inward edge of said first bus bar having a minor recess at said intersection away from the opposed bus bar; wherein an end of said second bus bar extends beyond said one end of said first bus bar, an elongate end portion of the inward edge of said first bus bar at said corresponding end being step recessed away from said second bus bar, said elongate end portion being mediate said intersection and said distal end and said minor recess at said intersection being recessed away from said second bus bar more than the step recessed end portion,     
     
     
       8. A substantially transparent, electrically heated vision unit comprising: a self-supporting substrate;   an electrically conductive film partially covering a surface of said substrate; and   a pair of opposed, elongate, electrically conductive bus bars, each having an elongate inward edge which faces the other and is in electrically conductive, overlapping contact with said film, a first bus bar of said pair of electrically conductive bus bars being spaced from, and substantially parallel to, a second bus bar of said pair of electrically conductive bus bars, one bus bar of said pair of electrically conductive bus bars extending from one end which forms an electrical junction with an electrically conductive lead, which lead is on a portion of said surface of said substrate which is not covered by said film and is electrically isolated from said film other than through said first bus bar, to a distal end, said electrical junction being on an area of said surface of said substrate not covered by said film, said one bus bar intersecting said film at an intersection mediate said one end and said distal end, said inward edge of said one bus bar having a minor recess at said intersection away from the opposed bus bar; wherein an end of said second bus bar extends beyond a corresponding end of said first bus bar, an elongate end portion of the inward edge of said first bus bar at said corresponding end being step recessed away from said second bus bar.     
     
     
       9. The electrically heated motor vehicle windshield of claim 8 wherein each said elongate end portion of said upper bus bar is step recessed away from said lower bus bar. 
     
     
       10. An electrically heated motor vehicle windshield having a generally trapezoidal shape, with a laterally extending lower edge, a laterally extending upper edge less wide than said lower edge, and right and left side edges extending from the lower to the upper edge, said windshield comprising: a substantially self-supporting, multi-ply, laminated substrate;   an electrically conductive film stack partially covering a laminated surface of a ply of said substrate, said film stack comprising an electrically conductive metal film sandwiched between dielectric films; and   a pair of opposed, elongated, electrically conductive bus bars on said laminated surface, each of said bus bars having an elongate inward edge which faces and is substantially parallel to the other and is in electrically conductive, overlapping contact with said film stack, an upper bus bar of said pair of bus bars being positioned proximate to said upper edge of said windshield, substantially parallel and substantially laterally coextensive therewith, and a lower bus bar of said pair of bus bars being positioned proximate said lower edge of said windshield substantially parallel and substantially laterally coextensive therewith, whereby said lower bus bar extends laterally at each end beyond said first bus bar, and elongate end portion of said inward edge at each end of said upper bus bar being recessed away from said lower bus bar, wherein said upper bus bar extends laterally to a distal end from a proximal end thereof which forms an electrical junction with an electrically conductive lead which is electrically isolated from said film stack except through said upper bus bar, said lead and said electrical junction both being on an area of said surface of said ply of said substrate not covered by said film stack, said upper bus bar intersecting a boundary of said film stack at an intersection mediate said proximal end thereof and the recessed elongate end portion at said proximal end thereof, said inward edge of said elongate end portion of said upper bus bar at said intersection being recessed away from said lower bus bar.   
     
     
       11. The electrically heated motor vehicle windshield of claim 10 wherein each end of said upper bus bar forms an electrical junction with a corresponding electrically conductive lead, which leads are electrically isolated from said film stack except through said upper bus bar, the leads and the electrical junctions being on an area of said surface of said ply of said substrate not covered by said film stack, said upper bus bar intersecting a right side and a left side boundary of said film stack at right side and left side intersections, respectively, each being mediate a corresponding end of said upper bus bar and the recessed elongate end portion at said corresponding end, said elongate end portion of said upper bus bar at each of said intersections being recessed away from said lower bus bar. 
     
     
       12. An electrically heated motor vehicle windshield having a generally trapezoidal shape, with a laterally extending lower edge, a laterally extending upper edge less wide than said lower edge, and right side and left side edges extending from the lower to the upper edge, said windshield comprising: a substantially self-supporting, multi-ply, laminated substrate;   an electrically conductive film stack partially covering a laminated surface of a ply of said substrate, having side boundaries each of which is substantially parallel to, and spaced from, a corresponding one of said right side and said left side edges of said windshield, said film stack comprising an electrically conductive metal film sandwiched between dielectric films; and   a pair of opposed, elongated, electrically conductive bus bars on said laminated surface, each of said bus bars having an elongate inward edge substantially laterally coextensive therewith which faces and is substantially parallel to each other and is in electrically conductive, overlapping contact with said film, an upper bus bar of said pair of bus bars being positioned proximate said upper edge of said windshield, being substantially parallel and substantially laterally coextensive therewith, and a lower bus bar of said pair of bus bars being positioned proximate said lower edge of said windshield, being substantially parallel and substantially laterally coextensive therewith, whereby said lower bus bar extends laterally at each end beyond said first bus bar, the inward edge of an elongate end portion at each end of said upper bus bar being step recessed away from said lower bus bar, said upper bus bar extends laterally to a distal end from a proximal end thereof, said proximal end forming an electrical junction with an electrically conductive lead which is electrically isolated from said film stack except through said upper bus bar, said lead extending to said electrical junction from a point proximate said lower edge of said windshield along a first one of the side edges of said windshield on surface area of said ply of said substrate not covered by said film stack, said upper bus bar intersecting a first one of said side boundaries of said film stack at an intersection mediate said junction and the step recessed end portin at said proximal end, the inward edge of said elongate end portion at said upper bus bar at said intersection being recessed, relative the step recess of said elongate end portion, away from said lower bus bar.   
     
     
       13. The electrically heated motor vehicle windshield of claim 12 wherein each said end portion is step recessed from about one-half to about one-eighth the lateral dimension of the end portion. 
     
     
       14. The electrically heated motor vehicle windshield of claim 12 wherein said upper bus bar has a lateral dimension of about forty-eight inches and the lateral dimension of each step recessed end portion of said upper bus bar is from about one to about three inches. 
     
     
       15. The electrically heated motor vehicle of claim 12 wherein said intersection is recessed a substantially semi-circular area having radius of about one-quarter inch. 
     
     
       16. The electrically heated motor vehicle windshield of claim 12 said distal end forms a second electrical junction with a second electrically conductive lead which is electrically isolated from said film stack except through said upper bus bar, said second lead extending from a second point proximate said lower edge of said windshield along a second one of the right side and left side edges of said windshield on surface area of said ply of said substrate to covered by said film stack, said upper bus bar intersecting a second one of said side boundaries of said film stack at a second intersection mediate said second electrical junction and the step recessed end portion at said distal end, the inward edge of the step recessed end portion at said distal end of said upper bus bar at said second intersection being recessed, relative the step recess of said elongate end portion at said distal end, away from said lower bus bar.